In recent years, a higher level of processing is being sought in the field of information signal processing, and with this trend, semiconductor integrated circuits that can process signals of a broader bandwidth have become necessary. In particular, transmission data rates have improved dramatically in optical communication systems, with optical transmission at transmission data rates of 2.4 gigabit/second (Gb/s) or 10 Gb/s having been put to practical use, and further, research and development progressing on systems having transmission data rates of 40 Gb/s or more. An optical receiver circuit is a circuit for converting an optical signal emitted from a transmission medium such as optical fiber to a current signal, and is made up from: a photodetector (i.e., a photoelectric conversion element) that converts an optical signal to a current signal, and a signal amplifier that converts the current signal from the photodetector to a voltage signal and then amplifies the voltage signal to the voltage required in circuits connected in subsequent stages.
FIG. 1 shows an example of the circuit configuration of an optical receiver circuit in the related art. Optical receiver circuit 400 shown in FIG. 1 includes: photodetector 402 converting optical signal 401 emitted from an optical fiber or the like to a current signal; preamplifier 403 connected to the output of photodetector 402 and made up from a transimpedance amplifier having feedback resistor 404; low-pass filter (LPF) 405 made up of resistor 406 having a resistance of R and capacitor 407 having a capacitance of C; and main amplifier 410 made up of two stages of differential amplifier circuits that are connected in cascade connection. LPF 405 passes the low-frequency component of the output signal of preamplifier 403 and delivers the low frequency component as a low-frequency signal. The signal amplifier is made up from preamplifier 403, LPF 405 and main amplifier 410. Each of preamplifier 403 and main amplifier 410 is a circuit constructed using FETs (field-effect transistors) as the basic transistors. A basic transistor is a transistor that relates to the operation of amplifying an electric signal as an active element in the amplifier circuit.
A semiconductor photodetector such as a photodiode is used as photodetector 402. The cathode of photodetector 402 is connected to power supply 414 for photodetector use, and a reverse voltage is applied to this cathode. The anode of photodetector 402 is connected to the input terminal of preamplifier 403. Photodetector 402 receives optical signal 401 emitted from an optical fiber or the like, converts optical signal 401 to a current signal (photocurrent) that corresponds to the optical signal, and supplies this current signal as output. In preamplifier 403, the current generated in photodetector 402 flows to feedback resistor 404, causing an electric potential difference across the two ends of feedback resistor 404, the output electric potential of preamplifier 403 changing in accordance with this difference. This operation of preamplifier 403 is referred to as “current-voltage conversion.”
Main amplifier 410 amplifies the voltage signal that is the output of preamplifier 403. Output OUTA of preamplifier 403 is led to one input terminal (non-inverted input terminal) INA of the first-stage differential amplification circuit of main amplifier 410. This signal supplied to non-inverted input terminal INA is referred to as the main signal. The low-frequency signal that has passed through LPF 405 from the output of preamplifier 403 is applied to the other input terminal (inverted input terminal) INAB of the first-stage differential amplification circuit of main amplifier 410. The low-frequency signal from LPF 405 gives an instantaneous average value for the signal supplied from preamplifier 403 over a time width that is of an order no greater than time constant T realized by resistor 406 and capacitor 407. The time constant T is represented by T=CR.
Main amplifier 410 amplifies the difference between two signal inputs, i.e., the main signal (the signal to non-inverted input terminal INA) that corresponds to data superposed on the optical signal and the instantaneous average value of the main signal (the signal to inverted input terminal INAB) and generates a complementary pair of outputs OUT, OUTB. Accordingly, in this optical receiver circuit, the polarities of outputs OUT, OUTB change according to whether the instantaneous value of the optical signal is greater or smaller than the instantaneous average value obtained from LPF 405. Using these complementary outputs OUT, OUTB in this way enables the implementation of processing that accords with the optical signal. Common power supply 411 is provided for preamplifier 403 and main amplifier 410, and constant-current source circuit 412 is provided for each differential amplification circuit in main amplifier 410.
In LPF 405, resistor 406 is provided between output OUTA of preamplifier 403 and inverted input INAB of main amplifier 410, and one end of capacitor 407 is connected to the connection point of resistor 403 and inverted input INAB and the other end of the capacitor is grounded. The cutoff frequency fc of this type of LPF 405 is given by
      f    c    =            1              2        ⁢        π        ⁢                                  ⁢        RC              .  Accordingly, resistance R of resistor 406 and capacitance C of capacitor 407 are determined by the frequency of the lower-frequency side of the data signal that is to be processed in this optical receiver circuit.
The optical receiver circuit shown in FIG. 1 is constructed by a signal amplifier that uses, as basic transistors, FETs in which current does not flow to the gate, but as shown in FIG. 2, an optical signal receiver circuit can also be constructed by a signal amplifier that uses bipolar transistors as basic transistors. Optical receiver circuit 420 shown in FIG. 2 is a circuit in which each FET in the signal amplifier of the optical receiver circuit shown in FIG. 1 is replaced with an NPN bipolar transistor. In other words, the circuit shown in FIG. 2 is a circuit provided with preamplifier 423 that uses bipolar transistors in place of preamplifier 403 shown in FIG. 1 and provided with main amplifier 430 that uses bipolar transistors in place of main amplifier 410 shown in FIG. 1. The signal amplifier is formed by preamplifier 423, LPF 405, and main amplifier 430.
In optical receiver circuit 420 including a signal amplifier that uses bipolar transistors as the basic transistors, despite the use of LPF 405 that is composed of resistor 406 and capacitor 407, the base current flowing to the bipolar transistors gives rise to a slight difference in the voltage level between the DC (direct current) level at non-inverted input terminal INA of the main signal and the instantaneous average value in the signal that is supplied from LPF 405 and applied to inverted input terminal INAB. In the case of receiver of a weak optical signal transmitted over a long distance, this voltage difference may obstruct the amplification of the signal. In other words, the receiver sensitivity may drop. As a method of solving this problem that arises from this difference, level-adjustment resistor 408 that has the same resistance as resistor 406 provided in LPF 405 is inserted between output OUTA of preamplifier 423 and non-inverted input INA of main amplifier 430 in the circuit shown in FIG. 2. However, when level-adjustment resistor 408 is inserted in the input path of the main signal to the differential circuit, the bandwidth of the signal amplifier for optical receiver circuit is narrowed as a whole, and in particular, gain drops on the higher-frequency side.
FIG. 3 shows the frequency characteristic of transimpedance gain for the signal amplifier as a whole when varying the value of level-adjustment resistor 408 of the signal amplifier, in optical receiver circuit 420 shown in FIG. 2. Here it can be seen that gain undergoes a large decrease on the higher-frequency side with increase in resistance R of level-adjustment resistor 408. As previously stated, the value of level-adjustment resistor 408, i.e., value R of resistor 406 of LPF 405, is determined by the frequency of the lower-frequency side of the data signal. For example, if the capacitance (for example 0.11 μF) that can be accommodated in a module that incorporates an optical receiver circuit is considered, when the cut-off frequency of the lower band is assumed to be 30 kHz, the necessary resistance R becomes 50Ω or more, but in a signal amplifier for an optical receiver circuit of the related art that includes level-adjustment resistor 408 of this magnitude, the transimpedance gain of the higher-frequency side in the signal amplifier falls precipitously.
Even in the case of a signal amplifier for an optical receiver circuit that uses bipolar transistors as the basic transistors, it is desirable to maintain high receiver sensitivity and high gain on the higher-frequency side.
As technology that relates to the present invention, JP-A-2000-031914 (Patent Literature 1) discloses an optical receiver circuit using an avalanche photodiode (APD) as a photodetector. In this optical receiver circuit, a component having a Bessel characteristic is used as an LPF provided between the preamplifier and main amplifier, and by detecting the output level of the main amplifier and varying the voltage applied to the APD in accordance with the detected level, fluctuation of the cut-off frequency and group delay is reduced over a broad dynamic range.
JP-A 2000-269590 (Patent Literature 2) relates to an optical transmission circuit and not to an optical receiver circuit, but discloses the provision of an emitter-follower circuit that receives a current signal corresponding to the optical signal to be transmitted and drives laser diode drive transistors by the output of this emitter-follower circuit.
JP-A-2003-051723 (Patent Literature 3) discloses an optical receiver circuit in which: a differential amplification circuit is used as a preamplifier directly connected to a photodetector; the first stage of a main amplifier is configured as an emitter-follower circuit and directly connects the output of the preamplifier with the input of the main amplifier; and further, an automatic offset-adjustment circuit including a low-pass filter is connected to the inverted input side of the preamplifier.
JP-A-H9-205331 (Patent Literature 4) discloses, in a differential amplification circuit in which a first differential amplification stage, a first emitter follower, a second differential amplification stage, and a second emitter follower are connected in that order in cascade connection, the insertion of a resistor between the first emitter follower and the second differential amplification stage.
JP-A-H9-233030 (Patent Literature 5) discloses the provision of an LPF between a preamplifier connected to a photodetector and a main amplifier.